The present invention relates to a liquid crystal display device that uses a liquid crystal material having spontaneous polarization and displays an image by driving a switching element to be on/off.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. Further, with the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices have been widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
By the way, the liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) positioned on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount that varies depending on environmental conditions, transmission type liquid crystal display devices are generally used as display devices of, particularly, personal computers displaying a multi-color or full-color image.
In addition, the current color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type, based on the liquid crystal materials to be used. The STN type liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the response speed. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a portable battery power source. Moreover, since a color image is displayed using a color filter, a single pixel must be constructed by three sub-pixels, and thus there are problems that it is difficult to achieve high definition and the purity of display color is not sufficient.
In order to solve the above problems, the present inventors et al. have developed a liquid crystal display device employing a field-sequential method. The liquid crystal display device employing a field-sequential method can easily realize a more definite display compared with a liquid crystal display device employing a color-filter method because it does not require sub-pixels, and it can also realize display color of excellent purity because the color of emitted light from a light source can be used directly for display without using a color filter. Moreover, since the utilization efficiency of light is high, the liquid crystal display device employing a field-sequential method has the advantage of low power consumption. However, in order to realize a liquid crystal display device employing a field-sequential method, a high-speed responsiveness of liquid crystal is essential. Therefore, in order to achieve a high-speed response of a liquid crystal display device employing a field-sequential method having the above-described advantage or of a liquid crystal display device employing a color-filter method, the present inventors et al. are carrying out the research and development on the driving of a liquid crystal, such as a ferroelectric liquid crystal having spontaneous polarization, which is expected to achieve a response at a speed 100 to 1000 times higher than a conventional speed, by a switching element such as a TFT (Thin Film Transistor).
In the ferroelectric liquid crystal, as shown in FIG. 1, the long axis direction of the liquid crystal molecules changes by only 20 with the application of a voltage. The intensity of transmitted light is changed by sandwiching a liquid crystal panel holding a ferroelectric liquid crystal therein by two polarizers whose polarization axes cross each other at a right angle and by using birefringence caused by a change of the liquid crystal molecules in the long axis direction. When the ferroelectric liquid crystal is driven by a switching element such as a TFT, the spontaneous polarization is switched according to the charge amount injected (stored) in a pixel through the switching element and the intensity of transmitted light changes.
By the way, in a conventional liquid crystal display device in which a liquid crystal such as a ferroelectric liquid crystal having spontaneous polarization is driven by a switching element such as a TFT, 2PSxc2x7A (the total charge amount of a switching current resulting from complete reversal of spontaneous polarization), where PS is the magnitude of spontaneous polarization per unit area and A is the electrode area of each pixel, is not larger than a charge amount Q to be charged in each pixel through a switching element. In other words, in order to satisfy the condition 2PSxc2x7Axe2x89xa6Q, the liquid crystal material, pixel electrode, TFT, etc. are designed.
In the conventional example, as described above, the cone angle 2xcex8 (xcex8: tilt angle) of the liquid crystal is set at 45xc2x0 or less, the spontaneous polarization is completely reversed under the condition of 2PSxc2x7Axe2x89xa6Q, and the maximum intensity of transmitted light is obtained. Therefore, with the application of a voltage of not higher than 7 V, the magnitude PS of spontaneous polarization that satisfies the above-described condition is reduced to 8 nC/cm2 or less, and a slow responsiveness will result because PS is not increased much, and there is demand for an increase in the magnitude of spontaneous polarization in view of the responsiveness, particularly the responsiveness at low temperature. Moreover, there is a problem that the degree of freedom in selecting a liquid crystal material is low. Due to the relation between the responsiveness and the selectable liquid crystal material, when a liquid crystal material having large spontaneous polarization is used, Q must be increased, resulting in a problem of an increase of the applied voltage. Furthermore, as shown in FIG. 2, at near end of the switching of spontaneous polarization, since the change of the optical axis due to the reversal of liquid crystal is small, the proportion of the change of the intensity of transmitted light due to the increase of the applied voltage becomes smaller, and therefore a high applied voltage is required in order to obtain the maximum intensity of transmitted light.
A principal object of the present invention is to provide a liquid crystal display device capable of using a liquid crystal material having large spontaneous polarization and decreasing the applied voltage to the liquid crystal material to a low voltage.
A liquid crystal display device of the first aspect is a liquid crystal display device comprising a liquid crystal material having spontaneous polarization between two substrates facing each other; and a plurality of electrodes and switching elements provided on an inner face of one of the substrates so that the electrodes and switching elements correspond to pixels, respectively, wherein a maximum charge amount injected to each pixel by the switching of each switching element is smaller than a total charge amount of a switching current per each pixel resulting from complete reversal of spontaneous polarization of the liquid crystal material.
A liquid crystal display device of the second aspect is a liquid crystal display device comprising a liquid crystal material having spontaneous polarization between two substrates facing each other; and a plurality of electrodes and switching elements provided on an inner face of one of the substrates so that the electrodes and switching elements correspond to pixels, respectively, wherein the liquid crystal display device satisfies a relation 2PSxc2x7A greater than Q, where Q is a maximum charge amount injected to each of the pixels by switching of each switching element, A is an area of each of the electrodes, and PS is a magnitude of spontaneous polarization per unit area.
Conventionally, a liquid crystal material, etc. is designed so that the magnitude PS of spontaneous polarization satisfies the condition 2PSxc2x7Axe2x89xa6Q, and the liquid crystal material is driven so that the spontaneous polarization is completely reversed by the application of a voltage. The present inventors et al. found as the result of studying in detail liquid crystal materials having spontaneous polarization, particularly the behavior of a ferroelectric liquid crystal driven by a TFT, that the ferroelectric liquid crystal can be driven by the TFT even in a condition reverse to the above-mentioned condition, that is, the maximum charge amount injected to each pixel by the switching of the switching element is smaller than the total charge amount of a switching current per pixel resulting from complete reversal of spontaneous polarization of the liquid crystal material, i.e., the condition 2PSxc2x7A greater than Q.
Accordingly, in the first and second aspects, an improvement of the responsiveness is achieved by designing a liquid crystal material, etc. to satisfy the condition that the maximum charge amount injected to each pixel by the switching of the switching element is smaller than the total charge amount of the switching current resulting from complete reversal of spontaneous polarization of the liquid crystal material, i.e., the condition 2PSxc2x7A greater than Q, so as to increase the spontaneous polarization. Moreover, by providing a display within a range in which the spontaneous polarization is not completely reversed, a portion where the proportion of the change of the intensity of transmitted light due to an increase of the applied voltage becomes smaller at near end of the switching of spontaneous polarization is not used for display, thereby realizing a decrease of the applied voltage.
A liquid crystal display device of the third aspect is based on the first or second aspect, wherein the cone angle of the liquid crystal material is not smaller than 45xc2x0. In the third aspect, since the cone angle of the liquid crystal material is not smaller than 45xc2x0, it is possible to obtain a more satisfactory characteristic. By making the cone angle not smaller than 45xc2x0, the maximum value is present in the relation between the applied voltage and the intensity of transmitted light as shown in FIG. 3. The switching angle of a liquid crystal material exhibiting this maximum value is substantially 45xc2x0. Therefore, it is preferred to set the cone angle of the liquid crystal material not to be smaller than 45xc2x0 and display an image in a region where the intensity of transmitted light is between substantially 0 and substantially maximum. In this case, compared with a conventional liquid crystal device in which the cone angle of the liquid crystal material is made 45xc2x0 by satisfying 2PSxc2x7Axe2x89xa6Q, in a liquid crystal display device of the present invention in which the cone angle of the liquid crystal material is made not smaller than 45xc2x0 by satisfying 2PSxc2x7A greater than Q, since a portion where the proportion of the change of the intensity of transmitted light due to an increase of the applied voltage becomes smaller at near end of the switching of spontaneous polarization is not used for display, it is possible to realize a decrease of the applied voltage.
A liquid crystal display device of the fourth aspect is based on any one of the first through third aspects, wherein the liquid crystal material is a ferroelectric liquid crystal material. In the fourth aspect, since a ferroelectric liquid crystal material is used as the liquid crystal material, it is possible to realize a high-speed responsiveness.
A liquid crystal display device of the fifth aspect is based on any one of the first through fourth aspects, wherein a storage capacity is provided on a liquid crystal material driving electrode side of each switching element. In the fifth aspect, with the presence of this storage capacity, it is possible to increase the maximum charge amount Q, thereby increasing the magnitude PS of spontaneous polarization per unit area.
A liquid crystal display device of the sixth aspect is based on any one of the first through fifth aspects, wherein a change of an optical axis of the liquid crystal material during driving is not greater than 45xc2x0. In the sixth aspect, since the change of an optical axis of the liquid crystal material during driving is not greater than 45xc2x0, it is possible to drive the liquid crystal material in a stable manner.
A liquid crystal display device of the seventh aspect is based on any one of the first through fifth aspect, wherein the liquid crystal material is driven within an applied voltage range of 0 to xc2x1V, where V is an applied voltage to the liquid crystal material when the light-transmittance of the liquid crystal material becomes maximum. In the seventh aspect, since the liquid crystal material is driven within the applied voltage range of 0 to V or xe2x88x92V to 0 (V: the voltage applied to the liquid crystal material when the light-transmittance becomes maximum), it is possible to drive the liquid crystal material in a stable manner.
A liquid crystal display device of the eighth aspect is based on the seventh aspect, wherein, if a charge amount injected to each pixel is denoted as q when the applied voltage is +V or xe2x88x92V, a total charge amount of a current flowing due to a response of the liquid crystal material resulting from the applied voltage +V or xe2x88x92V is not more than q. In the eighth aspect, since the liquid crystal material is driven so that the total charge amount of a current flowing due to a response of the liquid crystal is not more than q (q: the charge amount injected to each pixel when the applied voltage is xc2x1V) when the applied voltage is +V or xe2x88x92V, it is possible to drive the liquid crystal material in a stable manner.
A liquid crystal display device of the ninth aspect is based on any one of the first through eighth aspects and further comprises a back-light having a light source for emitting light of two or more colors, wherein a color image is displayed by switching the colors of emitted light of the light source in a time-divided manner in synchronism with on/off driving of the switching element. In the ninth aspect, by comprising the back-light having a light source for emitting light of two or more colors and by switching the colors of emitted light of the light source in a time-divided manner in synchronism with on/off driving of the switching element, it is possible to display a color image by a field-sequential method.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.